Excessive retardation by design is often a problem where wellbore temperature at the top of long liners is inadequate to insure a timely set of cement that has been retarded sufficiently to circulate past the higher wellbore temperatures present at the bottom of the well. Expensive remedial cement squeezes wherein the cement is retarded only enough to insure safe placement at the lower temperature present at the liner lap are typically the only means of insuring a proper cementation and hydraulic seal at the liner lap. An effective means of accelerating the cement set at the liner lap would eliminate the need for expensive remedial cement squeeze operations.
In squeeze cement applications it is desirable to set a retarded cement into open perforations and other void areas within the annular cement sheath or the formation to seal-off those formation intervals that are producing water and/or gas. Ideally the cement will set rapidly after placement. However, excessive retardation of the cement set often results from downhole contamination with the highly retarding drilling mud that may be present during the squeeze operation. The organic polymers and borates common to the fluids used to effect displacement or flushing of any excess cement from the wellbore at the end of some squeeze operations also create excessive retardation of the cement remaining in perforations or other squeezed areas. The risk of cement failure is increased by the longer set time as the cement remains vulnerable to flowback, gas or fluid-cut, and dislodgement until time of set. The 4-14 day WOC (Waiting-On-Cement) period that is common to achieve set after a coiled tubing squeeze operation produces additional operational costs. Hydrocarbon production is also lost or deferred during the extended WOC. It is desirable that a compatible accelerator be employed in some fashion to accelerate the set of the cement after placement.
The same is true when placing cement in the form of plugs in the wellbore. This may be for plugging back the well, forming pack-off or seal-off between formations and whipstock or kick-off operations. Cement plugs often fail to set within an allowable time span due to downhole commingling (contamination) with strongly retarding drilling fluids. Commingling results from placement mechanics and gravitational effects. It is desirable that an accelerator be employed to minimize or eliminate the set retarding effects and potential for downhole commingling.
Further, cement often becomes diluted with drilling mud or other downhole fluids due to gravitational exchange. Thixotropic or fast gelling cements are generally acknowledged to reduce this phenomena, but operational and temperature constraints often prevent their use in many cementing operations. It is desirable that a method be available to impart thixotropy or rapid gelation to the cement slurry after placement.